Present day telephone networks typically terminate customer premises equipment on a two-wire subscriber loop. The subscriber loop terminals to which customer premises equipment is connected are often referred to as “tip” and “ring”. The two-wires are used to receive an analog signal from a far-end transmitter and transmit an analog signal originating at the customer premises to the far-end.
Unfortunately, as bi-directional signals are carried on a single pair of wires, the two sets of signals can interfere with each other. In most applications, bi-directional signals on a single pair of wires do not present much concern. For voice calls, signals in both directions on the wire are simply perceived as two both parties speaking simultaneously.
Bi-directional signals, however, interfere destructively when the phone line is used to simultaneously carry data type signals, and voice. For example, phone lines are at times used to concurrently carry dual tone multi-frequency (DTMF) signals and voice signals. DTMF signals, for example, are typically used to control a customer premises alarm system from a remote monitoring station. Concurrently, a subscriber of the customer premises may speak to the monitoring station. Any portion of the signal that originates with the customer that is cross-coupled with the DTMF signal may interfere with the operation of a DTMF decoder at the customer premises.
Accordingly, local echo cancellation (often referred to as side-tone attenuation) circuits are known. Typically, however, side-tone attenuation circuits are interconnected to the tip and ring of the telephone line using a transformer and a balanced bridge incorporating precision resistors. The ideal bridge for any line is dependent upon the impedance of the line. Changes in the telephone line impedance causes an imbalance in the bridge and results in poorer side-tone attenuation in such circuits.
Therefore, there is a need for an improved side-tone attenuation circuit that is particularly useful in DTMF decoding circuits such as those used in alarm systems.